Misbah mentioned on TV the other day how Pakistani fans always care about who is not in the team, rather than who is in it. And it couldn't be more true.
Nawaz is a good cricketer, but is he a better option than Imad? Who has a proven track-record of success on the international level and has also been a better batsman than Nawaz.
Shadab's form may be up and down but he is the best limited-overs leg-spinner we have. The best fielder we have. Someone who can bat. Plus, in those conditions, there's no telling what a leg-spinner can do. And playing a diverse attack of a left-arm spinner, a leg-spinner and an off-spinner makes much more sense than going with two left-arm spinners: both of whom are similar in regards to how little they turn the ball.
Unless Shadab goes through a horrific run of form in the tournament, I don't see how he gets dropped. We've invested years in him and now its time to give him the proper opportunity to shine. If he goes through that horrific run of form then that's a worse case scenario and you go with Nawaz. But otherwise it does not make logical sense for Nawaz to replace Shadab or Imad; both bowers who aren't just better bowlers than him, but have also found more success in these conditions for Pakistan than him.